Ares could also be a strong antagonist. He might view the alliance between Annabeth and Zeus as a threat to his own plans for power and chaos. He could try to start wars or create discord to break their alliance and stop whatever they are working towards.
It could be Zeus himself. Given his powerful and often self - centered nature, he might oppose Annabeth's goals or try to manipulate her for his own ends. His actions could drive the conflict in the story.
A great antagonist could be a Titan who has a grudge against Zeus and sees Percy as an easy target to get back at him. For example, Atlas. He could try to use Percy to free himself from his burden again or trick Percy into doing something that would cause chaos among the gods. Another option is a powerful and vengeful demigod who feels that Percy, as the son of Zeus, has been given too much power and attention, so he wants to take Percy down.
It could be an old enemy of the Hunters. Maybe a powerful monster that has a grudge against them for past defeats. For instance, a giant who has been waiting for the right time to strike back at the Hunters.
A corrupted demigod who has turned to the dark side could also be the main antagonist. This demigod has made a deal with the forces of the House of Hades in exchange for power. They envy Percy and Annabeth's relationship and their status among the demigods, so they try to destroy them. They know all the secrets of the House of Hades and use this knowledge to constantly stay one step ahead of Percy and Annabeth.
It could be another god who is jealous of Zeus' interest in Atreus. Maybe a god who feels that Atreus is an outsider and doesn't belong in their divine affairs.
One possible plot could be that Annabeth, in her quest for knowledge, accidentally stumbles upon a long - forgotten secret of Zeus. Zeus, initially angry, realizes her pure intentions and decides to guide her on a journey to uncover ancient mysteries that could change the balance of power in the pantheon. This journey would test Annabeth's wits and courage as she has to deal with Zeus' unpredictable nature and the challenges that come with the knowledge they are seeking.
A powerful Titan who was once imprisoned by Zeus and now wants to destroy his legacy. This Titan could be seeking revenge for his long imprisonment and sees the legacy as a symbol of Zeus's power that he wants to crush.
It could be a powerful Titan who escaped from Tartarus and wants revenge on Zeus by targeting his son Percy. This Titan might have unique powers that make him a formidable foe.
A group of rogue demigods could be the main antagonists. They might be jealous of the attention the daughter gets because of her parents. So, they plot to make her life difficult and try to drive a wedge between her and the other demigods.
It could be an old enemy from their past, like a vengeful monster. Maybe a cyclops that they once defeated and now it wants to take revenge on them through their baby.